An image sensor on thinned silicon can be produced as follows: starting from a semiconductor wafer (silicon in principle, or silicon-on-insulator) on the front face before which operations are performed involving masking, implantations of impurities, depositions of layers of various provisional or definitive compositions, etching of these layers, heat treatment and so on; these operations make it possible to define an array of photosensitive pixels and electrical signal processing circuits associated with these pixels; the wafer is then bonded via its front face to the front face of a supporting substrate; most of the thickness of the semiconductor wafer is eliminated (such is the thinning operation), leaving on the front face of the support substrate a thin semiconductor layer comprising the photosensitive zones and the associated circuits; also, subsequently, various layers are deposited and etched on the rear face of the duly thinned semiconductor layer including, for example, an opaque metallic layer and a layer of color filters in mosaic form (for the color sensors).
It will be understood that, with this method, light does not arrive through a stock of insulating and conductive layers that might have been deposited (in CMOS technology, or another technology) on the photosensitive zones during the manufacturing of the semiconductor wafer. On the contrary, the light will arrive from the side of the rear face of the sensor, where appropriate pass through the color filters and directly reach the photosensitive zones without having to pass through the stack of insulating and conductive layers.
The residual thickness of the silicon after thinning is approximately 3 to 20 microns.
However, in practice, it is observed that there are crosstalk phenomena between adjacent pixels. The aim of the present invention is to further reduce the crosstalk defects, notably the colorimetry defects due to the crosstalk between adjacent pixels. This problem is all the greater when the surface area of the pixels is smaller.
The crosstalk can have optical causes (dispersion of photons to adjacent pixels as explained above) but also electronic causes; electronic crosstalk originates from the dispersion of electrons that are generated in a photosensitive zone corresponding to a determined pixel and that should be collected by an electrode corresponding to this pixel but that are in reality attracted by an electrode of a neighboring pixel because of the existence of electrical fields that are poorly controlled within the photosensitive zones.